A known sensor apparatus in which a circuit board is connected to a sensor through a cable detects the concentration of a particular gas component, such as oxygen or nitrogen oxides, by means of the sensor, and outputs an electric signal generated on the basis of the detection result from the sensor, to an external system from the circuit board (refer to, for example, Patent Documents 1 to 3). The circuit board of the sensor apparatus is generally accommodated within a housing so as to be protected from impact, water, and dust.